Forward more ports via Pagekite

[jernst]

Specifically:

• ssh
• the open ports listed in open-ports.d.

Given this might be a little tricky, we might need to implement this with a view on which apps we know of that open ports running with which protocols: they need to be supported pagekite protocols otherwise that won't work.

[jernst]

For now, let's just do ssh.
This requires a bit of experimenting per https://pagekite.net/wiki/Howto/SshOverPageKite/